Multiple Metrics with their own column pivots.

## What?
Add a possibility to add a column pivot only to certain metric and not to all metrics.

## Why?
The user would like to have a total rollup over a max of cpu processor usage. But since the
rollup will take all collumns and runs max on it he will not get a total but instead the max of all columns.

https://community.graylog.org/t/rollup-option-is-a-bit-useless/13747

## Your Environment
* Graylog Version: 3.2.1